Thailand: Cenozoic Basins and Structures

The Khlong Marui Fault (KMF) and Ranong Fault (RF) are major NNE trending strike-slip structures which dissect Peninsular Thailand. They crop out for 220 km and 420 km respectively, appear to pass into the Andaman Sea to the west and the Gulf of Thailand to the east, and are assumed to be conjugate to the NW trending Three Pagodas Fault (TPF) and Mae Ping Fault (MPF) in northern Thailand. According to the lateral extrusion model, a diachronous reversal in shear sense on the NW trending faults correlates to northward movement of the Indian indenter during the India – Eurasia collision. It follows that the KMF and RF are expected to show the opposite shear sense and an inversion at the same time as the TPF.

SRTM DEM of the Khlong Marui and Ranong Faults

Ian taking structural field measurements,Southern Thailand

Both faults are defined by elongate cores of metamorphic rocks with dextral shear fabrics, bound by brittle sinistral strands. Two phases of ductile dextral shear – D1 and D2 – are separated by Campanian inter-kinematic magmatism. Palaeocene to Eocene post-kinematic granites constrain the end of D2, while D3, the brittle sinistral phase, deforms these granites. Minor dextral deformation, D4, occurred at shallow levels, and obliquely truncates the older fabrics.

A metamorphic core complex at the northern end of the RF lies near the southernmost strand of the TPF. Its geometry is consistent with E-W extension, associated with D3. Basins at the ends of the fault are also orientated so that they should open during D3. Both faults splay at their southern ends as they enter the Andaman Sea, and may die out entirely before they enter the Gulf of Thailand. It seems likely that the faults did not drive extension in the basins, but acted as accommodation structures. Syn-extension sedimentation on both sides of the peninsula occurred from Mid Eocene to the end of the Oligocene, during D3 and core complex unroofing. Basin inversion during the Miocene correlates to the onset of D4.

The D1/D2 to D3 inversion is consistent with the faults’ hypothesised role as antithetic branches of the TPF. However, the sinistral phase on the TPF has previously been interpreted to occur 25 Ma after the equivalent D1/D2 interpreted here. This Upper Cretaceous – Palaeocene deformation is well before Himalayan continental collision, and may instead to be linked to variations in subduction rate around Sundaland’s SW margin. The Eocene – Oligocene D3 phase is likely to represent reactivation of the D1/D2 fabrics at a shallower depth in response to locally driven basin extension.